Attempts to use a photopolymerizable composition as an image-forming layer suitable for a photosensitive lithographic printing plate have been made for years. For example, the fundamental composition comprising an organic solvent-soluble polymer, an ethylenically unsaturated addition-polymerizable compound and a photopolymerization initiator described in JP-B-46-32714 (the term "JP-B" as used herein means an "examined Japanese patent publication"), the composition having an improved curing efficiency by introducing an unsaturated double bond into an organic solvent-soluble polymer described in JP-B-49-34041, and the composition including a novel photopolymerization initiator described in JP-B-48-38403, JP-B-53-27605 and British Patent 1,388,492 have been known. Some of these examples have been in practical use.
However, any of the photosensitive compositions used as a photosensitive layer of a lithographic printing plate suffers from polymerization inhibition owing to oxygen remarkably during image exposure, and therefore those photosensitive compositions have not been able to be practically used without providing an oxygen-intercepting layer comprising a water-soluble resin on the surface of the photosensitive layer. However, formation of the oxygen-intercepting layer results in high manufacturing cost and in deteriorated properties of the lithographic printing plate.
Thus, techniques for avoiding the influence of oxygen without providing the oxygen-intercepting layer comprising a water-soluble resin have been desired. As attempts to reduce the polymerization inhibition due to the influence of oxygen, behenic acid or wax is added to the above-mentioned photosensitive composition as an agent for forming an oxygen-intercepting layer, or a negative-working diazo resin is added thereto which is not so much affected by oxygen. Such attempts are described in, for example, JP-A-49-99790 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-54-95687, JP-A-59-206825, JP-A-59-53836, JP-A-59-178449, JP-A-57-196230, JP-A-63-63031, JP-A-63-206742 and U.S. Pat. No. 4,316,949. However, since any of these attempts has no effect sufficient for reducing the bad influence of oxygen, the lithographic printing plates obtained are affected by the degree of vacuum printing frame of an exposure machine in plate making, causing the properties (for example, sensitivity and impression capacity) of the lithographic printing plates to become unstable.
On the other hand, when images are formed on the surface of the photosensitive printing plate, a film original plate is generally superposed upon the photosensitive printing plate, which is exposed to light through the film original plate. At that time, in order to form more clear images, it is necessary to bring the photosensitive layer of the photosensitive printing plate into completely contact with the film original plate superposed to prevent the lack of sharpness in exposed images caused by the space between the both plates. In general, therefore, adhesion of the both plates is conducted by use of a vacuum printing frame (vacuum contact method), in which the photosensitive printing plate and the film original plate are placed between a glass sheet and a rubber sheet of the printing frame, and a gap between the glass sheet and the rubber sheet is evacuated, thereby bringing the both plates in close contact with each other. However, when the surface of the photosensitive printing plate is smooth, marginal parts of the both plates often adhere to each other at the beginning of evacuation to prevent central parts thereof from being thereafter evacuated. Accordingly, it takes a long time to contact closely the whole surface of the photosensitive printing plate with the film original plate, which uneconomically reduces the working efficiency of the plate making.
For the purpose of shortening the time required for the vacuum contact, in positive type photosensitive lithographic printing plates using o-quinonediazide and negative type photosensitive lithographic printing plates using a negative-type diazo resin, photosensitive printing plates containing a matting layer having a micro pattern in which coated portions removable on developing and non-coated portions are present in a mixed state, and processes for producing them have been developed and practically used, as described in JP-A-51-111102, JP-A-57-34558 (corresponding to U.S. Pat. No. 4,626,484) and JP-A-58-182636 (corresponding to U.S. Pat. No. 4,557,994).
However, the photosensitive lithographic printing plate prepared from the photosensitive composition to which behenic acid or wax is added as an agent for forming the oxygen-intercepting layer have disadvantage that the oxygen-intercepting layer reduces the adhesion of the matting layer, or the matting layer destroys the oxygen-intercepting layer, resulting in unstable properties (for example, sensitivity and impression capacity) of the lithographic printing plates.